1. Field of the Invention
The present invention relates to catheters, and more particularly to catheters with variable stiffness that may be used with an interchangeable core wire.
2. Background of the Invention
Cardiovascular disease, including atherosclerosis, is the leading cause of death in the U.S. The medical community has developed a number of methods and devices for treating coronary heart disease, some of which are specifically designed to treat the complications resulting from atherosclerosis and other forms of coronary arterial narrowing.
One method for treating atherosclerosis and other forms of coronary narrowing is percutaneous transluminal coronary angioplasty, commonly referred to as “angioplasty” or “PTCA”. The objective in angioplasty is to enlarge the lumen of the affected coronary artery by radial hydraulic expansion. The procedure is accomplished by inflating a balloon of a balloon catheter within the narrowed lumen of the coronary artery. Radial expansion of the coronary artery occurs in several different dimensions, and is related to the nature of the plaque.
Soft, fatty plaque deposits are flattened by the balloon, while hardened deposits are cracked and split to enlarge the lumen. The wall of the artery itself is also stretched when the balloon is inflated.
One or multiple dilations may be necessary to effectively dilate the artery. In many instances, multiple dilations using multiple “over-the-wire” balloon catheters having balloons with increasingly larger diameters maybe required. An over-the-wire catheter is one where a guidewire lumen is provided so that the catheter can be guided to the stenosis site by running the entire catheter length along the guidewire.
Conventional angioplasty guidewire typically include a proximal shaft, an intermediate section and a flexible distal tip. The proximal shaft comprises a solid wire or a solid wall tube. The proximal shaft primarily functions to guide and support a catheter, and to smoothly transmit rotation from the proximal end to an intermediate section.
The intermediate section of the guidewire extends axially from the proximal shaft and generally comprises a tapered core wire surrounded by a coiled spring and typically has more flexibility than the proximal shaft. Like the proximal shaft, the intermediate section must assist in guiding the catheter and smoothly transmitting rotation. However, some degree of flexibility in the intermediate section is desirable to conform the catheter to the curvature of the aortic arch and the coronary arteries.
A flexible distal tip mounts to the end of the intermediate portion and projects axially from the intermediate section. Typically, the flexible distal tip accepts a pre-formed curved shape resembling a “J”. The curved tip tends to steer the guide wire in the direction of the hook.
In a typical procedure, a physician will first insert and advance a guidewire to the stenosis site. An initial over-the-wire balloon dilation catheter having a fairly small diameter balloon is then passed over the guidewire to the site and the balloon is inflated to partially dilate the vessel. The balloon is then deflated and the catheter withdrawn. Balloon catheters having progressively larger balloons are then advanced to the stenosis along the guidewire, inflated, deflated, and then withdrawn in succession to sufficiently enlarge the lumen of the artery.
Balloon catheters include an inflation lumen through which a fluid can be forced to pressurize the balloon. As such, balloon catheters having a full-length guidewire lumen, must have at least two lumens. Catheters having more than one lumen are commonly referred to as “dual-lumen” or “multi-lumen” catheters.
Multi-lumen catheters have cross-sections in a variety of shapes. FIGS. 1 and 2 are examples of prior art dual lumen catheter cross-sections. FIG. 1 is a cross-section of coaxial catheter 100. Coaxial catheter 100 includes inner tube 102 and outer tube 104. Inner tube 102 defines an inner lumen or guidewire lumen 108 adapted to receive guidewire 106. Annular inflation lumen 110 is defined between inner tube 102 and outer tube 104, and is in fluid communication with an interior of a dilatation balloon (not shown).
In use, a guidewire is introduced into a coronary artery and is steered by manipulation of its proximal end, while being observed under a fluoroscope, until the guidewire passes through a stenosis in the artery. Once the guidewire is in place, a balloon dilatation catheter is advanced over the guidewire, being thus guided directly to the stenosis so as to place the balloon within the stenosis. Once so placed, the balloon is inflated under substantial pressure to dilate the stenosis.
The anatomy of coronary arteries varies widely from patient to patient. Often a patient's coronary arteries are irregularly shaped and highly tortuous. The tortuous configuration of the arteries may present difficulties to the physician in proper placement of the guidewire, and advancement of the catheter to the site of the stenosis. A highly tortuous coronary anatomy typically will present considerable resistance to advancement of the catheter over the guidewire.
With some types of catheter construction, the increased resistance may cause a tendency for portions of the catheter to collapse or buckle axially. For example, in a catheter having a shaft formed from inner and outer coaxial tubes, such as is shown in FIG. 1, and a balloon mounted to the distal ends of the tubes, there may be a tendency for the tubes to “telescope” when presented with an increase in resistance. The telescoping of the tubes tends to draw the ends of the balloon together slightly, but sufficiently to permit the balloon to become bunched-up as it is forced through the stenosis. The bunching-up of the balloon makes it more difficult for the balloon to cross the stenosis.
Additionally, it is sometimes necessary for the physician to place a torque load on the guidewire in an effort to overcome resistance encountered in a vessel. Torque is also used to steer the guidewire through separate passages and bifurcation of the anatomy. A torque load applied to a coaxial catheter can cause the outer tube to twist, while the inner tube remains stationary, causing a rotation of the tubes relative to one another.
FIG. 2 shows a cross-sectional view of a non-coaxial dual-lumen catheter 200. An inflation lumen 202 is in fluid communication with an interior of a dilatation balloon (not shown). A guidewire lumen 204 is defined at least in part by an inner tubular member 206 which extends the entire length of the catheter body. A guidewire 208 is shown within guidewire lumen 204. As explained above, catheter 200 is slid over guidewire 208 through a tortuous blood vessel.
Catheter 200 does not experience telescoping of the tubes upon increased resistance or twisting of the tubes relative to each other when a torque load is applied. However, once catheter 200 is selected and tracked over a guidewire inserted in a patient's vasculature, the physician may discover that the catheter has insufficient stiffness at its distal end to cross a lesion. This limits the use of such catheters in many procedures. Accordingly, a need exists for a physician to be able to change (for example, to increase) the stiffness of a catheter being used to traverse a particularly difficult lesion without removing the catheter from the patient's vasculature.